DESCRIPTION: (Investigator's abstract): The long term objectives of the present proposal is to study the physiology, and plasticity of excitatory synapses in the rat visual cortex. Specifically the aims of this proposal are: 1) Analysis of the excitatory synaptic input to non-pyramidal neurons in layer IV and to pyramidal neurons in layer II/III; 2) Analysis of the unitary connection between cells in the visual cortex; 3) Quantal Analysis of the EPSC; 4) Analysis of long term synaptic potentiation in layer IV and lI/III neurons. At the present, much is known about the anatomy of the visual cortex and about the light response characteristics of visual cortical neurons. It has been observed that synaptic connections in the visual cortex can undergo long term potentiation (LTP) following high frequency stimulation. However, because of technical difficulties, relatively little is known about the specific neurons that are involved in LTP and the mechanisms underlying its induction and expression. It is proposed here to use a new method of recording which allows visualization of individual neurons in live brain slices in combination with a high-resolution whole-cell recording. Stimulation of the thalamic afferents would be achieved by extracellular stimulation in close vicinity to the main dendrites. Stimulation of local excitatory neurons would be achieved by an intracellular stimulation of paired neurons. Under these conditions, the amplitude and subtype(s) of the unitary monosynaptic EPSCs would be determined. This data would than be used to examine the role of post-synaptic and pre-synaptic activation in inducing LTP. In addition, the change which occurs with LTP would be examined. This would be achieved by determining the EPSC quantal amplitude and by morphological measurements (using intensified epifluorescence) of the pre-synaptic terminal and post-synaptic dendritic spines before and after the induction of LTP.